1. Field of the Invention
The invention relates to a method for producing electrolyzed water, and more particularly to a method for producing electrolyzed water to be used for wet treatment of a semiconductor such as wet etching.
2. Description of the Related Art
A process of manufacturing a semiconductor device requires many wet treatments such as washing, etching and rinsing. In a process of manufacturing a semiconductor wafer, after a silicon ingot has grown, wet treatment is indispensable in a wafer slicing step and a mirror polishing step of a wafer in order to remove various contaminants such as silicon fine particles, abrasive powders and organic substances included in abrasive material. In such wet treatments there are used chemicals such as organic solvents, strong acid and strong alkali. In a process of manufacturing a semiconductor device, prior to wafer processing steps, it is necessary to use much of chemicals such as organic solvents, strong acid and strong alkali in many steps such as a Blanson washing step for cleaning a wafer with chemicals, and a photo-lithography step which further includes resist forming and removing steps, semiconductor layer forming and removing steps, insulating layer forming and removing step and metal layer forming and removing step.
As mentioned above, there are various wet treatments. The wet treatments used in a process for manufacturing a semiconductor device are grouped into three steps: a washing step, an etching step and a rinsing step. These three steps can be further categorized to the following three steps (A), (B) and (C).
(A) a substrate washing or rinsing step for removing contaminants adhered to the substrate without giving the substrate any influence. Herein, a substrate includes, for instance, a semiconductor layer, an insulating layer and a metal wire, and contaminants include, for instance, metal contaminants, organic or inorganic particles, residue of resist and ionic residue. PA1 (B) a step for etching a substrate PA1 (C) an etching step for removing natural oxide film or organic film formed on a surface of a substrate
For instance, in step (A) there are often used ammonium peroxide medium (APM) including NH.sub.4 OH,H.sub.2 O.sub.2 and H.sub.2 O at a ratio of 1:4:20 and sulfuric peroxide medium (SPM) including H.sub.2 SO.sub.4 and H.sub.2 O.sub.2 at a ratio of 5:1. In step (B), there is used hydrochloric peroxide medium (HPM) including HCl, H.sub.2 O.sub.2 and H.sub.2 O at a ratio of 1:1:6 as well as APM and SPM. In step (C), there is used dilute hydrogen fluoride (DHF) including HF and H.sub.2 0 at a ratio of 1:50 through 400 as well as APM and SPM.
Because of much use of chemical agents as aforementioned, it is required to have a plant for disposing of solid and liquid waste where the plant is expensive to run. The basic material used water. The used pure water is recycled by means of a closed system for reuse. Similarly, the chemical agents are also recycled for reuse. When the life of such recycled chemicals has expired and the chemicals are to be discarded, the chemicals are to be suitably decomposed and/or neutralized in order to prevent environmental pollution.
However, even if it is intended to reduce an amount of chemicals to be used by recycling, there is a limitation. In view of this problem, the inventors have invented an epochal method for wet treatment which is capable of remarkably reducing an amount of chemicals to be used, and have already filed a patent application with Japan Patent Office as Patent Application No. 5-105991 which was laid open to public on Sep. 16, 1994 a No. 6-260480. In this patent application, there is disclosed a method including the steps of electrolyzing pure water to which a quite small amount of electrolyte has been added, and washing a semiconductor with the thus obtained electrolyzed water. Hereinbelow, there will be explained an apparatus for carrying out the method disclosed in the above mentioned patent application, but it should be noted that the following explanation is only for better understanding of the present invention, and that the applicants do not admit the above mentioned patent application as prior art.
Hereinbelow, there will be briefly explained an apparatus for wet treatment with reference to FIG. 1. To an electrolytic cell 1, there is connected a conduit 12 through which the electrolytic cell 1 is supplied with pure water recycled through a water purifier 8 and an ion-exchanger 9 from waste liquid reservoir 7, and also with electrolyte to be added to the pure water through an electrolyte supplier 10. The electrolyte is added to the pure water for decreasing the specific resistance of the pure water. For instance, bubbling of carbon dioxide (CO.sub.2) and supporting electrolytic salt of ammonium acetate (CH.sub.3 COONH.sub.4) are used as an electrolyte source. The electrolytic cell 1 is divided into two sub-cells 1a and 1b by a partition membrane 2 composed of a material which does not allow water to pass therethrough, but allows ion to pass therethrough, such as porous silicon. In each of the sub-cells 1a and 1b is disposed an anode 3a and a cathode 3b each composed of platinum (Pt) or carbon (C). The anode 3a is connected to a positive terminal of a DC voltage source 5, while the cathode 3b is connected to a negative terminal of the DC voltage source 5. Electrolyzed water in each of the sub-cells 1a and 1b is introduced into disposal cells 6a and 6b, respectively. Waste solution discharged from the disposal cells 6a and 6b is gathered in the waste liquid reservoir 7, and then is purified in the water purifier 8, and further is recycled into pure water in the ion-exchanger 9. Each of the disposal cells 6a and 6b is provided with pH sensors 4a and 4b, respectively, for sensing H.sup.+ concentration or OH.sup.- concentration of the electrolyzed pure water. A pH regulator 11 receives signals representing H.sup.+ or OH.sup.- concentration from the pH sensors 4a and 4b, to thereby provide the electrolyte supplier 10 and the DC voltage source 5 with signals for controlling a DC voltage and an amount of electrolyte to be added to the pure water. Thus, pH of the electrolyzed pure water is maintained to be in a desired range.
Hereinbelow, there will be explained the method for wet treatment to be carried out using the above mentioned wet treatment apparatus. After pure water containing electrolyte has been provided to the electrolytic cell 1, across the anode 3a and the cathode 3b is applied a DC voltage intensive enough to generate an electric field having the electric field strength in the range of 10.sup.3 to 10.sup.4 V/cm, whereby the pure water is electrolyzed. The added electrolyte is being ionized to anion and cation in the pure water. Thus, when a DC voltage is applied to the anode 3a and the cathode 3b, the ionized anion and cation are attracted to the cathode 3b and the anode 3a, respectively, whereby an electrical current is generated across the electrodes 3a and 3b. The generation of an electrical current triggers electrolysis of the pure water. On a surface of the anode 3a is generated oxygen gas (O.sub.2), and thereby anodic water is generated. Herein, anodic water means water containing a high concentration of H.sup.+ ion therein. On a surface of the cathode 3b is generated hydrogen gas (H.sub.2), and thereby cathodic water is generated. Herein, cathodic water means water containing much of OH.sup.- ion therein. The anodic water is acidic, and is effective in the removal of contaminants contaminated with heavy metal, which is used to be performed by using SPM and HPM, and also in metal etching. The cathodic water is alkaline, and is effective in the removal of abrasive colloidal silica and residual chlorine ion, which is used to be performed by using APM. The used anodic and cathodic water is mixed with each other in the waste liquid reservoir 7, and is regenerated to pure water.
In accordance with the method mentioned so far, it is possible to remarkably reduce the use of chemicals such as acid and alkali with the result of smaller amount of waste. Thus, compared to a conventional method, the method makes it possible to reduce an amount of waste and cost for disposing of waste, and also makes it unnecessary to build a waste disposing plant, with the result of lower manufacturing cost of semiconductors. Thus, large economic advantages can be expected.
In "Washing Design", Spring Edition, 1987, published by Kindai Hensyusha Syuppan, an article titled "Redox washing method: New washing method in electronic industry" has suggested a method in which electrolyzed water is used, for instance, for silicon etching and for removing oxide film formed on a surface of aluminum layer. The suggested method includes the steps of electrolyzing pure water or tap water including a low concentration of electrolyte, and etching silicon by using the obtained cathodic water. In accordance with the method, impurities adhered to or dispersed on a surface of silicon together with the surface of silicon can be removed by etching. If aluminum wiring is immersed in replace of silicon, the oxide film formed on a surface of the aluminum wiring can be removed without overetching the aluminum wiring.
When the above mentioned apparatus is to be used in an experiment, only a small amount of electrolyzed water is needed per unit period of time. However, when the apparatus is to be used in an actual semiconductor manufacturing step, it is necessary to further increase the ability to produce electrolyzed water. Recently, 8 inch wafers have been often used. For instance, if it is intended to cleanse 1 lot including fifty 8 inch wafers by a batch method, then it is necessary to prepare at least 50 liters of electrolyzed water. However, if it takes too long produce a required amount of electrolyzed water throughput goes down, and further there is a fear that oxidation reduction potential (hereinafter, referred to simply as "ORP") of the electrolyzed water may be changed. For increasing the production of electrolyzed water only, the electrolysis area may be increased or a larger voltage may be applied to electrodes. However, if a larger voltage is to be applied to the electrodes, it would be necessary to install a high voltage generating apparatus and an apparatus for safety, and also be necessary to often exchange electrodes because the life of electrodes is made shorter due to a larger voltage applied thereto, thereby increasing the cost for such apparatuses. In addition, the battery exchanges tend disadvantageously to introduce particles and other contaminants, which are the most deadly foe to the wet treatment apparatus, into the wet treatment apparatus. On the other hand, if the electrolysis area is to be enlarged, greater amount of electrolyzed water can be produced, but the consumption of electrical power is also increased. Thus, the above mentioned method cannot further increase the production efficiency of electrolyzed water.
Furthermore, the production of electrolyzed water (anodic water or cathodic water) has been controlled by monitoring a pH value, that is, H.sup.+ concentration, or a pOH value, that is, OH.sup.- concentration. However, the inventors have discovered the fact that ORP, which means a potential representing the intensity of oxidation or reduction, may vary though a pH value hardly varies at all in the case of electrolyzed water. In addition, the inventors have also discovered the fact that ORP is a parameter which is capable of varying independently of a pH value.
FIG. 2 is a graph showing successive change of a pH value and ORP over time go by when anodic and cathodic waters are stored for 0 through 140 hours in two polyethylene containers A and B of openings having different diameters. The container A has a barrel body having about 10 cm of diameter and has a narrow opening having 1.5 cm of diameter, while the container B has a barrel body having about 10 cm of diameter and has an opening having the same diameter as that of the body. Each of the containers A and B is filled with electrolyzed water, and is hermetically sealed with a cap. As a result of this experiment, the inventors discovered that ORP remarkably varies though the pH value only slightly varies. The cathodic water varies more remarkably than the anodic water, and ORP varies in a different way in each of the containers A and B. For instance, when the cathodic water having pH 10.5 and -800 mV ORP was contained in the container B, the pH value was kept almost constant for approximately 140 hours, but ORP returned to approximately 0 mV after only an hour. On the other hand, when the cathodic water was contained in the container A, it took approximately 70 hours for ORP to return to approximately 0 mV. The pH value stayed constant. Though the reason why ORP varies in different fashion in dependence on the containers is not known; it is considered as follows. Though the containers are hermetically sealed with a cap for isolating the contents from the atmosphere, the cap is removed from the container when ORP is measured. Thus, the contact area of the electrolyzed water with the atmosphere when a probe is inserted into the electrolyzed water is varied in each of the containers. The difference in contact area between the containers having different opening diameters may influence ORP. The anodic water varied more slightly than the cathodic water, but, as is shown in FIG. 2, ORP decreased from approximately 1200 mV to approximately 1100 mV in 70 hours. The pH value stayed at approximately 1.5, and did not change. Thus, it was understood that only ORP was varied in the anodic water.
In view of the above mentioned results, when electrolyzed water is to be used for wet treatment, electrolyzed water has to be prepared taking successive change of ORP into consideration. For instance, even though much time is spent preparing an adequate amount of cathodic water for electrolysis in order to remove colloidal silica located on silicon, if a lot to be treated arrives too late for some reason, the prepared electrolyzed water would be deteriorated and hence could not be used. In such a case, electrolyte and electrical power consumed to produce the electrolyzed water have been wasted. Though electrolyzed water of ORP having a large absolute value may be produced taking the deterioration of electrolyzed water into consideration, the consumption of electrical power is disadvantageously increased. Thus, it is desired to enhance the production efficiency of electrolyzed water so that an adequate amount of electrolyzed water can be produced in a short period of time in order to timely prepare electrolyzed water.
The above mentioned article included in "Washing Design", Spring Edition, 1987, merely reports that a surface of silicon can be etched or oxide film of aluminum can be removed by immersing a silicon piece or aluminum wiring in cathodic water. In addition, when pure water is to be electrolyzed, electrolysis cannot be carried out unless a high voltage is applied to electrodes because pure water has high resistivity, specifically, 18 M.OMEGA..multidot. cm. Furthermore, it is necessary to use special equipment for applying a high voltage. If pure water contains electrolyte even in a small amount, it is possible to decrease the voltage to be applied. However, documents known in the art merely teach electrolyte such as tap water, ethylenediaminetetraacetic acid (EDTA) which is effective in removal of fingerprints, and citric acid. These electrolytes are all unpreferable or the most deadly foe to a semiconductor device, and hence cannot be used. In the above mentioned patent application filed by the inventors, only electrolyte giving no influence to a semiconductor device is selected for use, but the production efficiency of electrolyzed water is still dissatisfactory. If a voltage is not be increased and an amount of added electrolyte is increased, the production efficiency of electrolyzed water can be enhanced. However, in such a case, it is impossible to reduce the amount of chemicals to be used.